3GPP LTE (Long-Term Evolution) systems support both FDD and TDD.
FIG. 1 is a schematic diagram illustrating a frame structure of an FDD system.
As shown in FIG. 1, in an FDD system, each radio frame (101) has a length of 10 ms, and includes 10 subframes. Each subframe (103) has a length of 1 ms, and contains two time slots (105) each of which lasts 0.5 ms, i.e., the k'th subframe contains time slot 2k and time slot 2k+1, k=0, 1, . . . , 9.
FIG. 2 is a schematic diagram illustrating a frame structure of a TDD system.
As shown in FIG. 2, in a TDD system, each radio frame (201) of 10 ms is divided into two equal half-frames each of which lasts 5 ms. Each half-frame (203) includes 8 time slots each of which lasts 0.5 ms, and 3 special fields, i.e. Downlink Pilot Time Slot (DwPTS) (211), Guarding Period (GP) (213) and Uplink Pilot Time Slot (UpPTS) (215). The 3 special fields altogether last 1 ms. Each subframe (205) is composed of two consecutive time slots (207), i.e., the k'th subframe includes time slot 2k and time slot 2k+1. A downlink transmission time interval (TTI) is defined in a subframe.
A TDD system supports 7 types of uplink/downlink (UL/DL) configurations, as shown in Table 1. In the table, D denotes a downlink subframe, U denotes an uplink subframe, S denotes a special subframe including the 3 special fields.
TABLE 1LTE TDD UL/DL configurationConfigura-tionSwitch-serialpointSub-frame IDnumberperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The frame structures shown in FIG. 1 and FIG. 2 are ideal frame structures of LTE systems. In practice, a base station and a UE may have different timing for sending and receiving subframes due to propagation delay. In an FDD system, timing of UL/DL subframes of a base station are generally aligned. In a TDD system, a time interval is generally added between a UL subframe and a DL subframe to allow the base station to transit from a receiving state to a sending state. According to LTE TDD standards, the time interval is 20 us, i.e., timing for receiving a UL subframe by a base station is 20 us prior to ideal subframe timing.
FIG. 3 illustrates a method of determining starting timing of sending a UL subframe by a UE.
The UE takes timing of a DL signal received from the base station as a reference for determining UL starting timing. Due to propagation delay, the UE needs to advance the transmission of the UL signal by a certain time period to guarantee the UL signal of the UE satisfies a required timing relation when received by the base station. The time advance (TA) (301) of the UE is (NTA+NTA offset)×TS seconds. TS is a sampling interval obtained by using a sampling frequency of 30.72 MHz. In an FDD system, NTA offset equals 0, and the base station adjusts transmission TA of a UE by adjusting the value of NTA to make boundaries of UL subframes and DL subframes aligned at the base station. In a TDD system, NTA offset equals the value of 624, NTA is configured by the base station. The TA actually used by the UE is (NTA+NTA offset)×TS seconds, so that the timing of receiving the UL subframe at the base station is 20 us prior to the ideal timing of TDD subframes, which provides time for the base station to transit from receiving to sending.
In an LTE system, a UE triggers a random access process by sending a random access preamble signal when attempting to access the system. The UE determines starting timing of preambles of physical random access channel (PRACH) preamble formats 0-3 by taking NTA=0. As such, for an FDD system, the starting timing of a PRACH preamble signal is directly obtained by using the timing for receiving DL signals from the base station; for a TDD system, the starting timing of a PRACH preamble signal is 20 us prior to timing for receiving DL signals from the base station. With respect to PRACH preamble signal format 4, the UE determines timing of the end position of the UpPTS time slot by taking NTA=0, i.e., sending the preamble signal 4832 TS and 20 us prior to the timing of receiving DL signals from the base station.
In an LTE-A (LTE-advanced) system, multiple CC (component carriers) are aggregated to obtain larger working bandwidth, i.e., CA (carrier aggregation). The aggregated carriers constitute downlink and uplink links in the communication system, therefore larger transmission rates can be achieved. A base station may configure a UE to work in multiple Cells which include a Pcell (Primary Cell) and multiple Scells (Secondary Cell). According to LTE Release 11, it is configured that HARQ-ACK of all Cells that are configured to be received by the UE is fed back in a UL subframe in a Pcell.